Earth, tu, is the pivotal phase of the Wu Xing, and its placement differs from the Western scheme in an important way: earth is associated not with a single season but with the centre, and with the transitions between seasons, the late-summer or the doubling days that link each season to the next, so that it functions as the balancing hub around which the other four phases turn. It is linked to the colour yellow, the planet Saturn and the qualities of stability, nourishment and mediation. In the generating cycle earth is produced by fire and gives rise to metal (ores form in the ground); in the controlling cycle earth is restrained by wood and itself dams and controls water.
In Chinese medicine earth governs the spleen and stomach, the emotion of worry or pensiveness, the mouth and the flesh, and it is central to digestion and the transformation of food into energy. Earth's symbolism is of the centre, of grounding and nurture, of fidelity and reliability, the fertile, receiving ground and the still point that harmonises the cycle.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown